I'm new to Fallout 3, and on arriving at Megaton, talked to a beggar outside asking for clean water, which I didn't have (only slightly radioactive). He said they had clean water for sale in Megaton. I said I'd get him some. I spent two days of game time in Megaton, and probably explored 80% of it, making friends with various people including the water purifier engineer and the stores, saloon, and restaurant shopkeepers, talking to almost everyone (the clinician too), and looking at almost every bottle and cup and water source... I didn't see any clean water anywhere.
Was the beggar just wrong that there's water for sale there?